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I'm doing that. Trust me, I'm not just whining here. This place is just for venting. ;)
I had same issue with the apple music playlists. To fix I logged out of my account on iPhone and signed in again. Those songs you can't remove are only on that device. I won't mess with playlists again until apple fixes.
 
There are a handful of albums that I have "made available offline" that do not show up when I select "Show Music Available Offline."
 
Definitely bugs with offline mode. Periodically it will just stop working. I'll switch to offline mode but all tracks will still be shown. If I force quit the app and restart, then it will only show offline tracks. Bug has been reported.
 
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